I. Field of the Invention
The present invention relates to a two level inverter circuit for operating a gaseous discharge lamp, and more particularly, to an emergency lighting circuit for operating a gaseous discharge lamp from a DC energy source upon failure of the primary AC source, the circuit being provided with two power level operation to conserve energy in the DC source.
II. Description of the Prior Art
Electric power failures due to inclement weather conditions and equipment breakdowns have been a plague for many years. A power failure, no matter what may be the cause, may very well jeopardize human life due to lighting system failure. There are, therefore, many installations which require some type of emergency lighting system which will automatically come into operation upon the occurrence of a power failure. The high efficiency of a fluorescent lamp makes it especially valuable for use in an emergency lighting system.
Presently available emergency lighting systems are generally of the type using transistor switching inverter circuitry. In a typical arrangement, a single lamp (or group of lamps) is used for both normal AC operation of the lighting system and for the emergency system, a rechargeable battery being the power source for energizing the transistor inverter when the AC line voltage fails. A principle limitation of such emergency lighting systems resides in battery capacity, that is, the amount of lamp operating time a given battery will provide. To be taken into consideration is the fact that a higher power level is required to start such a lamp than is required to maintain it in operation once started. This especially is the case in an arrangement wherein the lamp is eight feet in length, and over. Operating the lamp after starting at the same power level as is required to effect starting results in the operating life of the battery, and therefore of the system, being less than recommended.
It is desirable therefore to provide an emergency lighting system capable of maximizing the operating time on a given battery charge.
Accordingly, it is an object of the present invention to provide an emergency lighting system having an inverter capable of operating on two power levels: a high power level for starting the lamp and a lower power level for running the lamp after starting.